The present invention relates to a structure and a method of making a light emitting diode (LED) chip, and more particularly to a structure and a method of making an AlGaInP LED chip.
The conventional AlGaInP LED, as shown in FIG. 4, has a double heterostructure (DH), which is consisted of an n-type (AlxGa1-x)0.5In0.5P lower cladding layer 4 with an Al composition of about 70%xcx9c100%, formed on a n-type GaAs substrate 3, an (AlxGa1-x)0.5In 0.5P active layer 5, a p-type (AlxGa1-x)0.5In0.5P upper cladding layer 6 with an Al composition of about 70%xcx9c100% and a p-type high energy gap GaAsP, InGaP, AlGaP, GaP, or AlGaAs current spreading layer 7. The emitting wavelength of the conventional LED structure can be changed by adjusting composition of the active layer to generate a wavelength changed from 650 nm red to 555 nm pure green. One disadvantage of the conventional LED is that, when the light generated by the active layer is emitted downward to the GaAs substrate, the light will be absorbed by the GaAs substrate since the GaAs substrate has a smaller energy gap. Accordingly, the light-output performance of the LED will be greatly reduced.
There are some conventional LED technologies have been disclosed in order to avoid the absorption of light by the substrate. However, these conventional technologies still have some disadvantages and limitations. For example, Sugawara et al. disclosed a method, which has been published in Appl. Phys Lett. Vol. 61, 1775-1777 (1992), that adding a distributed bragg reflector (DBR) layer on the GaAs substrate so as to reflect the light emitted downward to the GaAs substrate and to decrease the light absorbed by the GaAs substrate. However, because the DBR layer only reflects light that is of near normal incidence to the GaAs substrate, so that the efficiency is not very great.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1-x )0.5In0.5P/GaP light emitting diode [Appl. Phys Lett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1-x)0.5In0.5P/GaP]. This TS AlGaInP LED was fabricated by growing a very thick (about 50 xcexcm) p-type GaP window layer using hydride vapor phase epitaxy (HVPE). Before bonding, the n-type GaAs substrate was selectively removed using chemical mechanical polishing and etching techniques. The exposed n-type (AlxGa1-x)0.5In0.5P claddng layers are subsequently wafer-bonded to 8-10 mil thick n-type GaP substrate. The resulting TS AlGaInP LED exhibits a two fold improvement in light output compared to absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology [Appl. Phys Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding]. They used the AuBe/Au as the adhesive to bond the Si substrate and LED epilayers. However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcd with 20 mA injection current and is still 40% lower than the luminous intensity of TS AlGaInP LED.
As described above, the conventional LED has many disadvantages. Therefore, the present invention provides a LED structure and method of making the same to solve the conventional disadvantages.
The present invention provides a light emitting diode. A light emitting diode, the light emitting diode comprises a LED epitaxial structure having a multi-layered AlGaInP epitaxial structure formed on a light-absorbing substrate; a transparent substrate; and a layer of transparent adhesive material for bonding the transparent substrate and the multi-layered AlGaInP epitaxial structure. The active layer of the LED can be composed of single heterostructure (SH), double heterostructure (DH), multi quantum wells (MQWs), or quantum wells heterostructure (QWHs). Meanwhile, a first and a second ohmic contact metal layer are bonded to a first and a second conductive-type epitaxial layers, respectively. Besides, both the first and second ohmic contact metal layers are located on the same side.
The present invention provides a method for manufacturing a light emitting diode, which comprises the steps of: providing a LED epitaxial structure having a multi-layered AlGaInP epitaxial structure formed on a light-absorbing substrate; providing a transparent substrate and using a layer of transparent adhesive material, for example, BCB (B-staged bisbenzocyclobutene) resin or Epoxy resin, to bond the transparent substrate and the multi-layered AlGaInP epitaxial structure. The light-absorbing substrate is then removed to expose the first conductive-type etching stop layer so that a first ohmic contact metal layer is, for example, formed. The etching step also exposes the second conductive type epitaxial layer to form a second ohmic contact layer. In addition, both the first and second ohmic contact metal layers are located on the same side.
An advantage of the present invention is to provide a simple LED structure, the adhesion process of the LED structure can be performed at lower temperature to avoid the evaporation problem of V group elements. Moreover, by the use of the transparent substrate, the light emitting efficiency of the LED can be significantly improved.
Another advantage of the present invention is the simplified process, wherein the low cost glass can be used as the material of the transparent substrate. Accordingly, a throughput with high yield and low cost is achieved.
Another advantage of the present invention is the use of the elastic properties of transparent adhesive material to bond the transparent substrate and the multi-layered AlGaInP epitaxial structure. Therefore, an excellent boding result can be obtained by the use of the elastic transparent adhesive layer even if the epitaxial structure has a roughness surface.